The present invention is directed to the field of optical signal processing and in particular to the processing of interferometric signals utilising a coupler or the like.
Sagnac interferometric techniques are often utilised in various measurement fields including optical gyroscopes and current measuring devices. For example, PCT Application No. PCT/AU92/000680 by Clarke discloses one form of current sensor device utilising interferometric techniques. These devices utilise a coupler for coupling optical signals into and out of the overall Sagnac arrangement. The basic form of a current sensor according to Clarke includes a Sagnac loop attached to a 3xc3x973 coupler with the coupler being utilised to couple optical signals in and out of the Sagnac loop. An optical source injects light via the coupler bidirectionally around the Sagnac loop with the output on each arm of the coupler being measured by detectors.
The arrangement utilising the 3xc3x973 coupler is used to measure rotation (gyroscope) or magnetic fields (current sensor). The arrangement is a passive interferometer in that it contains no active optical elements for modulating frequency, phase or amplitude. In such a passive system, the stability of the optical components is critical. Unfortunately, in practice the 3xc3x973 coupler has been known to change its coupling parameters with temperature and time.
It is an object of at least preferred embodiments of the present invention to provide for an improved form of interrogating a Sagnac interferometer.
In accordance with a first aspect of the present invention, there is provided a method of interrogating an optical coupler interconnected to a two beam interferometer, so as to determine an idealised output value, said optical coupler having at least three bidirectional external input/output ports, said method comprising the steps of: for each of said external input/output ports: inserting an optical signal into the port, measuring a corresponding output at each of said ports, forming summations of those corresponding outputs which, in an ideal coupler, would have equal intensities; and determining said idealized output value of said coupler from said summations.
The optical coupler can comprise a 3xc3x973 coupler and each of the summations can be formed from a corresponding output from each of the ports for a particular optical signal insertion.
The two beam interferometer can comprise a Sagnac interferometer.
The two beam interferometer can comprise a Michelson interferometer.
In one implementation, the summations are formed from a single optical pulse appropriately delayed so that said corresponding outputs arrive at a detector in a temporally spaced manner.
In accordance with a second aspect of the present invention there is provided an apparatus for interrogating an optical coupler interconnected to a two beam interferometer, so as to determine an idealised output value, said optical coupler having at least three bidirectional external input/output ports, said apparatus comprising: means for inserting optical signals into each of the ports; means for measuring a corresponding output at each of said ports when an optical signal is inserted into one of the ports; and means for forming summations of those corresponding outputs which, in an ideal coupler, would have equal intensities.
The apparatus may further comprise means for determining the idealised output value of the coupler from said summations.
Devices implementing the method are also disclosed.